Physics midterm question: book held against vertical wall

Physics 205A Midterm 1, fall semester 2015
Cuesta College, San Luis Obispo, CA

An applied force pushes horizontally on a book with just enough force to still keep it stationary against a vertical wall that is not frictionless. Discuss why the book will begin to slide downwards if the magnitude of the applied force is slightly decreased. Explain your reasoning using free-body diagram(s), the properties of forces, and Newton's laws.

Solution and grading rubric:

• p:
  Correct. Complete free-body diagrams, and discusses/demonstrates:
  1. the applied force F_applied on the book to the right is equal in magnitude to the normal force N of the wall on the book to the left, due to Newton's first law (F_applied = N);
  2. for the case of the stationary book, the upwards static friction force f_s is at its maximum value μ_s⋅N, and is equal in magnitude to the downwards weight force w, due to Newton's first law (μ_s⋅N = w);
  3. for the case of the book beginning to slide downwards, lessening the applied force decreases the normal force, which decreases the maximum static friction force to less than that of the weight force (which remains constant), such that there is a downwards net force (Newton's second law), and the book will accelerate downwards.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Two of the three points (1)-(3) correct/complete.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying Newton's laws to a free-body diagram.
• x:
  Implementation of ideas, but credit given for effort rather than merit. No systematic application of Newton's laws to the forces on a free-body diagram.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855, 73320
Exam code: midterm01j0r6
p: 22 students
r: 15 students
t: 18 students
v: 19 students
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 4105):
A sample "x" response (from student 1002):